How does the Mounted Combatant feat interact with Infernal War Machines?

Question

The new Baldur's Gate - Descent into Avernus hardcover features a brand new mechanic: Infernal War Machines! A few things caught my eye - first off, that they have a stat block:

An infernal war machine's statistics function like those for creatures

Then, that their opportunity attack mechanics function a lot like those of mounts:

Infernal war machines provoke opportunity attacks as normal. When an infernal war machine provokes an opportunity attack, the attacker can target the vehicle or any creature riding on or inside if that doesn't have total cover and is within reach.

Also, that their driver can control their movement much like a mounter controls a (non-intelligent) mount's movement:

While the vehicle is moving, the driver can steer it along any course.

And that they can make the vehicle Dash/Disengage (in a similar but different manner that a mounter commands a mount to do so):

As a bonus action, the driver can do one of the following: […] Cause the infernal war machine to take the Dash or Disengage action while the vehicle's engine is running.

In summary, Infernal War Machines seem similar, in certain points at least, to mounts. What I'm wondering here is: how would a driver with the Mounted Combatant feat interact with these Machines? Would the feat's benefits work with the Machines or not?

Reminder on the Mounted Combatant feat:

You are a dangerous foe while mounted. While you are mounted and aren't incapacitated, you gain the following benefits:

• You have advantage on melee attack rolls against any unmounted creature that is smaller than your mount. You can force an attack targeted at your mount to target you instead.
• You can force an attack targeted at your mount to target you instead.
• If your mount is subjected to an effect that allows it to make a Dexterity saving throw to take only half damage, it instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.

From a thematic standpoint, I think it would make sense for a specialized "Helldriver", so to speak, to gain benefits from that feat while driving an Infernal War Machine, but actual-mechanics-wise, I am unsure.

Answer 1

Specific Rules Apply - RAW

By Rules as Written, a mount is a mount. A mount follows the rules for mounts in the Player Handbook/Basic Rules/SRD. Infernal Machines are from Descent and don't use mount rules, even if the rules are similar, they are different things.

Mounted Combatant Feat - Houserule

While this, RAW, doesn't apply to infernal machines, as above, I don't think that it would be game breaking for a feat with almost identical rules to exist/house rule for infernal machines. I've seen a game with pistols that applied this idea to Crossbow Expert. I think if the new thing is similar enough there isn't harm in trying it -- with strict explanation to the players that since it is off book, you reserve the right to alter or amend the houserule.

I'd be temped to make it its own feat as expertise in riding a horse in real life doesn't make you a good driver, and being a NASCAR driver doesn't make you good at riding a horse. Moreover, if the house rule is found to be problematic for infernal machines you can tweek just the language of the new feat without messing with mounts. Remember, Jeremy Crawford explains the 5e philosophy as "rules of exceptions" based on the idea that new stuff shouldn't break or change old stuff.

Answer 2

No you cannot "mount a vehicle"

On page 198 of the PHB it states:

A willing creature that is at least one size larger than you and has an appropriate anatomy can serve as a mount.

On page 216 of BG: DiA it states:

Infernal war machines are vehicles built in the Nine Hells and fueled by the souls of the damned.

PHB page 181-182 have a section titled "mounts and vehicles". The two are kept as different subjects. The Mounted Combatant Feat and the lance both specify the term "mounted". Even though they have similar text does not make an infernal war machine a mount.